Long-Lasting Extreme Magnetic Storm Activities in 1770 Found in Historical Documents

TL;DR

This study uncovers evidence of an exceptionally prolonged and intense geomagnetic storm in September 1770, based on historical documents and sunspot observations, revealing insights into extreme space weather events of the past.

Contribution

It presents new historical evidence of a prolonged, extreme magnetic storm in 1770, with detailed analysis of auroral displays and sunspot data, expanding understanding of past geomagnetic storm durations.

Findings

Auroral displays lasted nearly 9 nights at low latitudes.

Sunspot area was twice as large as in the 1859 storm.

The storm involved multiple large magnetic structures ejected sequentially.

Abstract

Dim red aurora at low magnetic latitudes is a visual and recognized manifestation of geomagnetic storms. The great low-latitude auroral displays seen throughout East Asia on 16-18 September 1770 are considered to manifest one of the greatest storms. Recently found 111 historical documents in East Asia attest that these low-latitude auroral displays were succeeding for almost 9 nights during 10-19 September 1770 in the lowest magnetic latitude areas (< 30{\deg}). This suggests that the duration of the great magnetic storm is much longer than usual. Sunspot drawings from 1770 reveals the fact that sunspots area was twice as large as those observed in another great storm of 1859, which substantiates this unusual storm activities in 1770. These spots likely ejected several huge, sequential magnetic structures in short duration into interplanetary space, resulting in spectacular world-wide aurorae in mid-September 1770. These findings provide new insights about the history, duration, and effects of extreme magnetic storms that may be valuable for those who need to mitigate against extreme events.